There are certainly times when a pilot needs to have someone with them to allow them fly. For instance an NPPL holder with the most restrictive medical needs a safety pilot to be able to carry pax.
Or during line training in an airline, there needs to be a safety pilot.
This is all unloggable time. The person is just there in case the proverbial hits the rotating object. Not as a cheap way to build hours.
In each example you give, it shows the reason why these a/c are only single crew, yes there were two pilots onboard, but they weren't trained for multi crew operations and in each case this was a factor that contributed in someway to the accident.
An airliner cannot be flown single crew unless there is an situation such as incapacitation and whilst it is possible to fly something like a 737 solo, certainly if the skipper has a heart attack, then how is the F/O to taxi the thing whilst on the ground.
Your average spamcan does not require two pilots, though as many PPL's fly together, there is usually some delegation of tasks between them, but at no time can two PPL's log the same time in an a/c. They could log half the flight each, but not the whole thing or even one log it all and the other log 50%.
In an emergency such as crew incapacitation, then normal rules don't really apply. Certainly what to log would be the last thing on your mind. Though it would probably be fair that the person who took over and did all the work was able to log the time they did, but that would mean the incapacitated pilot wouldn't be able to log it if it were a single crew a/c.
What is asked here, though is about pre-meditated fudging of hours or "pencil time". It would be very difficult to say what was an acceptable level of "help" on a flight that would allow two pilots to log the time. So to keep it sensible. an a/c that only REQUIRES a single pilot, only one person can log it, whereas a machine designated as multi pilot, needs more than one person to operate it on a normal basis, so the crew must always log it as there is clear and defined tasks for them to do to keep the a/c pointed in the right direction. Not the case with a single crew a/c.